JoThelan already said it, but I have to stress how common this is in swahili-Culture. Often people start yelling "hodi hodi" when they are close to the house, before coming up to the door. The reply is always "Karibu/ni!" =Welcome/plural! or Karibu ndani! = Welcome inside!
Saying "knock" or "knock knock" is an integral part of the cultures of Africa I have experienced. It is a way of asking for permission to enter one's homestead or door without physically knocking on something. Other examples,
Ndau = dododo , reply is "gumai" literally meaning "arrive" Shona = gogogoi, reply is "svikai" literally meaning "arrive" Ndebele = qoki qoki, reply is "ngenani" literally meaning "enter"
I am hoping others might share something about the etymology, but it's not really clear to me that it's not onomatopoeic. Looking at animal "sounds" from a variety of languages, many are explained as just that - "the sound the animal makes". But they can be very different! Except for often being repeated. "Hav hav! Wolf wolf! Oink oink! Ghrutu ghrutu!" Etc.
As in orange - Swahili "o" is always open/rounded [ɔ] https://en.wikipedia.org/wiki/Open-mid_back_rounded_vowel